Calendar style changes for Dark Theme

-) Day Header and Today Selection didn't look good with Dark Theme
-) Replaced Color Resources with Brush Resources where Possible
-) Replaced Button Template with Style, because they where affected by external style changes

(Left new, right old)
![2013-10-07 15_08_54-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280476/f705ce00-2f52-11e3-83b2-d6b2aa8329dc.png)![2013-10-07 15_17_26-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280477/f705f696-2f52-11e3-8c25-557bb392e66e.png)